With age, skeletal muscle shows a progressive loss of muscle mass leading to muscle weakness in the elderly. Transcription factors play important roles in tissue development and maintenance by regulating the expression of genes required for cell function. Changes in gene expression accompany the aging process in both muscle and other tissues. Age related alterations in the transcription level of various genes could be due to selective changes in transcription factor binding properties and/or levels of expression during aging. NFAT (nuclear factor of activated T cells) is a transcription factor originally identified in T cells which plays a prominent role in regulation of cytokine gene expression. Subsequently, mRNA for various NFAT isoforms have been shown to be expressed by non-lymphoid cells, including skeletal muscles. The role of NFAT isoforms in non-lymphoid cells is unknown. However, pharmacologic agents which block NFAT-mediated transcription in skeletal muscle cells have profound effects on muscle cell growth and differentiation both in vivo and in vitro thereby demonstrating a critical role for NFAT proteins in skeletal muscle. The overall goals of this proposal are as follows: 1) Using mice in which the NFAT4 isoform has been "knocked out", we will evaluate whether this isoform is necessary for the growth, differentiation and/or maintenance of skeletal muscle cells in vivo and in vitro. 2) Using direct DNA injection of the NFAT-responsive reporter constructs into skeletal muscle, we will determine if aging leads to decreases in NFAT-mediated transcription in skeletal muscle. Our long term goal is to study the signal transduction pathway of specific NFAT isoforms in skeletal muscle. Towards that end we are interested in identifying the receptor- ligand pairs and signal transduction pathways which activate NFAT in skeletal muscle cells as well as the genes which are regulated by NFAT. Age-related changes in the NFAT system may occur in skeletal muscle thus leading to decreases in the expression of proteins with a role in maintaining muscle mass.